A 14,500-year record of atmospheric 14C contains evidence for 7000- and 3500- year oscillations. Since the Holocene archaeomagnetic record also contains features consistent with a 7000-year cycle, a likely explanation is that the 14C data are recording modulation of cosmogenic isotope production due to variations in geomagnetic shielding. A longer-term 36Cl record contains evidence for similar periodicities, suggesting that these geomagnetic variations may have been present for tens of millennia.